One of the most difficult problems related to air pollution control of boilers and incinerators is that of controlling the oxides of nitrogen (NO.sub.x). The NO.sub.x emissions that result from the high temperature oxidation of nitrogen in combustion air are to a certain extent, controllable by in-furnace techniques of combustion modification and flue gas reburning. However, the 50% NO.sub.x reduction from these techniques is insufficient for compliance with Phase II Reasonable Available Control Technology (RACT) of the 1990 Clean Air Act Amendments requirements. As a result, the industry is now looking toward flue gas treatment technologies (FGT) to control NO.sub.x emissions, especially inasmuch as such processes are useful to achieve higher removal efficiencies. The FGT technologies are broadly classified as dry and wet techniques; dry techniques are further classified as selective catalytic reduction (SCR),selective noncatalytic reduction (SNCR), adsorption, and electron beam irradiation. Both dry catalytic processes and wet scrubbing processes have been applied in Japan for the treatment of NO.sub.x effluents from power plants. As of the late 1980s, more than 100 utility boilers in Japan had been equipped with SCR units. The SCR processes which operate at elevated temperatures are very difficult to apply to pollution sources containing oxides of sulfur, including sulfur dioxide, and particulates. The scrubbing processes require expensive oxidizing agents and, because of the high concentration of chlorides and nitrates in the spent scrubbing solution, present disposal problems. Recently, several studies have reviewed the use of nonthermal plasma technologies have been of interest for the control of NO.sub.x emissions. However, although previous results show that 90% of NO conversion is achievable, between 40% to 60% of the converted NO emerges from the reactor as NO.sub.x. Thus, the total NO.sub.x destruction is only 55%--only a slight improvement over combustion modification technologies. In 1993, thorough review of nonthermal plasma reactors for NO.sub.x treatment was compiled and published by B.M.Penetrante.
"Control of NO.sub.x Emissions by Reburning", Summary Report [CERI, National Risk Management Research Laboratory, Office of Research and Development, U.S. Environmental Protection Agency (Febuary, 1996)] defines reburning as combustion modification, wherein the formation of NO.sub.x is minimized in one portion of the boiler and a portion of the NO.sub.x that is destroyed in another NO control employing reburning technology is described in 1996 as a new, effective method of controlling NO.sub.x emissions from a wide range of stationary combustion sources including large, coal-fired utility boilers. NO.sub.x emission control technologies that are capable of achieving NO.sub.x emission reduction from a coal-fired boiler are classified as either combustion modifications or post-combustion flue gas treatment. Combustion modification techniques prevent the formation of NO.sub.x during combustion or destroy the NO.sub.x formed during primary combustion. These techniques include the use of low-NO burners (LNBS), overf ire air (OFA), and boiler combustion optimization. Post-combustion flue gas treatment reduces the NO.sub.x content of the flue gas through techniques such as selective catalytic reduction (SCR) and selective noncatalytic reduction (SNCR).
Unlike some other NO.sub.x controls, for a wide variety of boilers, it is possible to implement reburning technology within a relatively short period of time. Reburning technology is particularly applicable to wet bottom (i.e.,slagging) boilers. For these boilers, the only other commercially available NO.sub.x control is flue gas treatment, which is more costly per ton of NO.sub.x reduction achieved.
It is known that reburning reduces NO.sub.x emissions by completing combustion in three stages. In the first stage, NO.sub.x formation resulting from interactions at high temperatures of the fuel and the combustion air at high temperatures is controlled by reducing the burner heat release rate and the amount of oxygen present. In the second stage, additional fuel is added under reduction (oxygen-deficient) conditions to produce hydrocarbon radicals that react with the NO.sub.x formed in the first stage nitrogen gas (N.sub.2). Additional combustion air is added in a lower-temperature third stage and combustion is completed.
Besides the influx of reburning technology, nonthermal plasma has gained significance in the control of gaseous pollutants, especially NO.sub.x emissions, sulfates SOX (oxides of sulfur) emissions, and volatile organic compounds (VOCs), see A. Mizuno et al., Reactive Absorption of NO.sub.x Using Net Discharge Plasma Reactor [IEEE Transactions of Industry Applications Vol. 31, No. 6, (November/December, 1995)]. In NO.sub.x removal, the conventional selective catalytic reduction (SCR) method and the recently developed electron beam irradiation (EB) method both use ammonia injection to convert NO.sub.x into ammonium nitrate. For small scale plants or vehicles using diesel engines, as ammonia injection presents some safety hazards, this technique is not advisable. Alternately, investigations on nonthermal plasma processing indicates that the plasma created by electrical pulse discharge is suitable for controlling the NO.sub.x and soot in diesel engine exhaust. In the pulse discharge, NO gets converted to NO.sub.2 which is absorbed by a liquid film maintained in the reactor. Such semiwet reactors are also adaptable for the collection of fine particles. In the application of the wet-type reactors discussed in this paper, it is suggested that about half of NO removed by the plasma dissociates into N.sub.2 and O.sub.2 and the rest is absorbed by water. The study also indicates that wet-type reactors performed better than the dry-type reactor in the removal of NO.sub.x.
Nonthermal discharge devices are further reviewed by G. E. Vogtlin et al. in an article entitled "Pulsed Corona Discharge for Removal of NO.sub.x from Flue Gas," NATO ASI Series, Vol. G34, Part B [Springer-Verlag: Berlin, Heidelberg (1993)]. In this review, many types of nonthermal discharge devices used for environmental applications are discussed. All of the devices considered, operate on the same principle and produce a discharge in which a majority of the electric energy goes into the production of energetic electrons, rather than into gas heating. The authors found that although the energetic electrons are short-lived at atmospheric pressure and rarely collide with a pollutant molecule, many collisions occur with the dominant background gas molecules. The radicals produced, in turn, decompose the toxic compounds. The efficiency of the approach arises from the radicals having long lifetimes and reacting selectively with the contaminant molecules. The review article fund that these reactions create microdischarges, which yield large improvements in the power of efficiency. This results because, within the short lifetime of each microdischarge, the ions do not experience significant movement and do not contribute to power consumption. The short lifetime of these microdischarges is accomplished with the use of very-short voltage pulses (pulsed corona discharge) and/or with the use of dielectric coatings on the electrodes (dielectric barrier discharge).
Although there has been an extensive review of the technical literature, no system presently teaches toward the removal of substantially all the NO.sub.x content of flue gas emissions or combining of the pulsed corona discharge technology and of the chemical scrubber technology. The combination of these technologies is inhibited by the production during the pulsed corona discharge operation of ozone and other by-products, which products create hitherto unsolved technical problems. The ozone and other byproducts scavenge the NO.sub.2 absorbent, such as sodium sulfite, and depletes the scrubbing solution. This results in the scrubber failing to absorb NO.sub.2 from the effluent of the plasma discharge section. Under normal conditions, in the application disclosed hereinbelow, the plasma discharge generates an ozone concentration of about 5 to 10 times that of the concentration of the NO.sub.x in the effluent being treated. When ozone is present in such concentrations, because of the reactivity thereof, the ozone dominates the competition for the NO.sub.2 absorbent in the scrubbing solution. Therefore, the scrubbing medium, e.g. sodium sulfite solution, is not efficiently used for NO.sub.2 absorption. Additionally, the plasma unit creates by-products by oxidizing some of the nitric oxide, NO, to an NO.sub.3.sup.- anion, which becomes a strong acid in solution and reduces the pH in the scrubbing solution. As the scrubbing solution is basic, the additional acidic input neutralizes and decomposes the sodium sulfite. It is further noticed that the use of corona treatment of effluents generates a certain amount of heat, which increases the temperature in the scrubber and causes the decomposition of the scrubbing medium. Finally, plasma discharge, especially DC and pulse plasmas, collect particles which accumulate in the scrubbing solution and cause plugging problems in the packed bed scrubber. The system disclosed for the control of NO.sub.x emissions, infra, overcomes these problems and further, does so in a way which complies with RACT standards.